1. Field of the Invention
The present invention relates to a developing unit for visualizing an electrostatic latent image and, more particularly, to a developing unit capable of forming a high-quality image using a monocomponent developer and an image forming apparatus having the developing unit.
2. Description of the Related Art
A developing unit used in an image forming apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus, as disclosed in Japanese Laid-open Patent Publication (Kokai) No. 61-169859 has a hopper containing toner. The toner is supplied from the hopper to an electrostatic latent image formed on a photosensitive drum by rotation of a developing roller. A toner supply roller, which rotates in the same direction as that of the rotation of the developing roller, is brought into contact with the developing roller to supply the toner to the developing roller. Futhermore, the end of a doctor blade serving as a thin-layer-forming member is brought into contact with the developing roller by uniform pressure, and a thin layer of the toner is formed on the developing roller. The thin layer is made close to or placed into contact with the photosensitive drum by the rotation of the developing roller; thus the electrostatic latent image is made visible.
In a conventional developing unit having the above structure, toner cannot be supplied sufficiently to the developing roller and, if non-magnetic toners are used, they cannot be supplied by a magnetic force. For this reason, when a solid image is formed, the density of the trailing edge of the image is lower than the leading edge thereof. When a plurality of solid images are formed in sequence, the density of the images is depressed or a toner layer is not formed completely. Futhermore, over time, toner gets into a surface layer of the toner supply roller and the surface layer hardens. For this reason, the capability of transfering toner of the toner supply roller is decreased and the condition of the toner layer gets worse.